It is known that the detection and the evaluation of the severity of certain diseases are based on the analysis of echographic images. Thus, intravascular ultrasound imaging has been developed for characterizing atheromatous lesions in man. The resolution which can be obtained with ultrasound depends in fact on the frequency used, and the higher this frequency, the better the resolution. However, the attenuation which the ultrasound beam suffers increases with the frequency, and the depth of penetration of the beam diminishes considerably. An image with very good resolution at high frequency can therefore only be obtained by the intravascular route.
The treatment of the lesions due to the presence of atheromatous plaque in the coronary arteries involves recourse to two distinct invasive techniques: surgery and catheter intervention (balloon angioplasty or laser). The use of catheterized instruments represents a comfortable alternative. However, surgery proves necessary when the lesions are serious, that is to say when they are in an advanced state. In this case, the surgical interventions consist in replacing the stenosed portion of the coronary artery with part of another artery, either by removal and grafting, which creates a bypass round the zone, or by using another artery to irrigate the heart.
The catheters are used for the interventions employing the balloon angioplasty technique. In this technique, a catheter measuring about 1.5 meters in length and 1 mm in diameter is introduced percutaneously into the femoral artery and is advanced to the coronary artery by progressing through the arterial network. The probe is guided with the aid of a guide wire which may or may not be integral with the catheter and whose end is curved in order to avoid perforations during the manoeuvres needed for advancing and orienting the catheter. This progress is monitored by an external imaging technique, which makes the positioning easier and safer. The therapy can also be based on the abrasion of the atheromatous plaque by laser or mini-scalpel, the guiding of which is based on the same principle as for the angioplasty catheters.
The criteria for the choice of using one or other of the methods mentioned hereinabove are empirical. They take account of a priori knowledge of the structure of the plaque and of the extent of its thickness and length. Despite the preference for angioplasty, because of its greater simplicity, the two methods nevertheless entail substantial risks.
The manoeuvrability of the catheter, the simplification of the intervention procedures, the increase in the resolution cell and the awareness of the position of the distal end of the catheter are the key points in the development of this therapeutic control technique.
Intracoronary echography is distinguished by the small dimensions of the elements to be investigated and of the routes for gaining access to these. This is reflected in a quite particular specification for the design of the catheters, the piezoelectric elements, the elements permitting scanning, and the instruments for treating the stenoses.
Furthermore, the aim of quantitative echography is the complete exploitation of the echographic signal and the extraction of the quantitative parameters aiding in the diagnosis and providing additional information compared to the traditional echographic image. Thus, the methods for characterizing tissues by ultrasound permit estimation of the quantitative parameters on the basis of the echographic signal or echographic image. These methods also demand specific characteristics as regards the systems for obtaining the echographic signals or images.
Thus, the cable of the catheter, the total length of which is about 1.50 meters, has a proximal part which is sufficiently rigid to permit guiding, and a more flexible distal part which has an overall diameter of 1 to 2 mm and a length of 2 to 15 cm. The distal end is made up of a capsule which is transparent to ultrasound and which contains the echographic system immersed in a coupling liquid. The assembly must be compatible with the biological media, and polymers such as silicone, polyvinylchloride, polyethylene, polyurethanes, polyesters, polytetrafluoroethylene, are generally used for its production.
The central frequency of the wide-band transducers used varies from 20 to 40 MHz and their supply voltage is between 100 and 300 V.
The catheterized ultrasound probes used for intracoronary echography can be classified in two ways: according to the mode of scanning permitting construction of the image, and according to the treatment instruments installed in the catheter.
The first solution for constructing an image in a sectional plane consists in making the piezoelectric element turn. The principal characteristic of this solution lies in the need to have electric contacts between a mobile part, consisting of the transducer, and a fixed part, consisting of the external medium. Moreover, the distance separating the ultrasound source from the tissues to be investigated is small, hence the risk of positioning in the near-field zone. To solve the latter problem, use is made of a rotating mirror, either plane or concave, which deflects the ultrasonic waves through 45.degree., and this makes it possible, by means of the transducer emitting in the axis of the catheter, to lengthen the distance between the emitter and the tissues. Whilst maintaining this structure, the transducer can be fixed to the catheter, with the mirror being the only installed element.
This latter solution is the one most commonly used. However, the mobile assembly consisting of a transducer and of a mirror is used for positioning the catheter after an independent guide wire has been introduced.
To illustrate the state of the art in this field, the following may be mentioned in particular:
U.S. Pat. No. 4,794,931, which describes a catheter for echography within a cavity, the distal end of which is equipped with an ultrasound transducer, this transducer being able to be driven in rotation by an external motor in order to guarantee the scanning by the beam. PA1 WO 93/05712, which also describes a catheter for echography within a cavity, the distal end of which includes a fixed ultrasound transducer, a mirror for deflecting the ultrasonic waves and oriented at 45.degree., and a micromotor which is of the timepiece type and on whose shaft the said mirror is mounted by a posteriori assembly. PA1 the length of the catheter and the passage of the latter through cavities of curved geometry generating a variable torque, there is no control of the angular position or speed of rotation of the distal end of the catheter, and hence of the member for emission and acquisition of the ultrasonic waves, PA1 the variations in the speed of rotation cause distortion of the image, PA1 the catheter conveys a mechanical shaft whose rigidity is inimical to mobility through the sinuous arterial network, additionally presenting the risk of dissection of the atheromatous plaque, PA1 the large diameter of the distal end of the catheter permits only a reduced investigation of the coronary arteries, limited to the proximal zones, where the stenosis does not greatly reduce the lumen of the vessel, PA1 the lack of manoeuvrability of the mechanical scanning devices due to the connection to an external rotation module, PA1 the rigidity of the distal end of the catheters motorized internally, due to the substantial length of the micromotor and of its shaft (greater than 5 mm); this part not being flexible and rendering the positioning of the catheter dangerous (perforation, detachment of atheromatous plaque), PA1 the use of a micromotor of the electromagnetic type requires double electromagnetic compatibility with the biological environment and the distal capsule of the catheter: both for the electric field and for the magnetic field.
The systems currently known for catheterized echography have the following faults in particular: